LTC1773EMS データシートの表示（PDF） - Linear Technology

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LTC1773EMS

Synchronous Step-Down DC/DC Controller

Linear Technology 

LTC1773

APPLICATIONS INFORMATION

PC Board Layout Checklist

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of the

LTC1773. These items are also illustrated graphically in

the layout diagram of Figure 6. Check the following in your

layout:

1) Are the signal and power grounds segregated? The

LTC1773 signal ground consists of the resistive divider,

the compensation network and CSS. The power ground

consists of the (–) plate of CIN, the (–) plate of COUT, the

source of the external synchronous NMOS, and Pin 5 of

the LTC1773. The power ground traces should be kept

short, direct and wide. Connect the synchronous

MOSFETs source directly to the input capacitor ground.

2) Does the VFB pin connect directly to the feedback

resistors? The resistive divider of R1 and R2 must be

connected between the (+) plate of COUT and signal ground.

Be careful locating the feedback resistors too far away

from the LTC1773. The VFB line should not be routed close

to any other nodes with high slew rates.

3) Does the (+) terminal of CIN connect to VIN as closely as

possible? This capacitor provides the AC current to the

external power MOSFETs.

4) Keep the switching nodes SW, TG and BG away from

sensitive small-signal nodes, especially from the voltage

and current sensing feedback pins.

CC2

RC

CC1

1 ITH

10

SW

CSS

R1

2 RUN/SS SENSE– 9

3

LTC1773 8

SYNC/FCB VIN

4

VFB

7

TG

5

GND

6

BG

R2

Design Example

As a design example, assume the LTC1773 is used in a

single lithium-ion battery powered cellular phone applica-

tion. The VIN will be operating from a maximum of 4.2V

down to about 2.7V. The load current requirement is a

maximum of 2A but most of the time it will be on standby

mode, requiring only 2mA. Efficiency at both low and high

load currents is important. Output voltage is 2.5V. With

this information we can calculate RSENSE to be around

33mΩ. For the inductor L, using equation (1),

( )( ) L =

f

1

∆IL

⎛

VOUT ⎝⎜1–

VOUT

VIN

⎞

⎠⎟

(3)

Substituting VOUT = 2.5V, VIN = 4.2V, ∆IL = 800mA and

f = 550kHz in equation (3) gives:

L

=

2.5V

550kHz (800mA)

⎛

⎝⎜1–

2.5V ⎞

4.2V ⎠⎟

=

2.3µH

A 2.5µH inductor works well for this application. For good

efficiency choose a 4A inductor with less than 0.1Ω series

resistance.

CIN will require an RMS current rating of at least 1A at

temperature and COUT will require an ESR of less than

0.066Ω. In most applications, the requirements for these

capacitors are fairly similar.

RSENSE

D1

Q1

Q2

+

+

CIN

VIN

–

BOLD LINES INDICATE

HIGH CURRENT PATHS

COUT

L1

Figure 6. LTC1773 Layout Diagram

VOUT

1773 F06

1773fb

13

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